The best replacement for mylar is white paint.

The best alternatives to mylar are rated as following:
★★★★★  white paint
★★★★★  vellum
★★★★  polypropylene
★★★★  aluminum foil
★★★  acetate
★★★  foil
★★★  latex balloons
★★  kapton
★★  ceramic capacitor
★  vacuum bags
★  panda film
★  mirrors
★  flat white
★  melinex
★  kevlar
★  ceramic capacitors
★  panda plastic
  dacron
  white poly
  electrolytic capacitors
  tedlar
  flat white paint
  aluminized polyester

Your alternative for mylar is not listed? Suggest!
Speaking about mylar, one probably shouldn't forget about foodsaver.

Other search terms related to mylar and their ratings are:
★★★★★ foodsaver
★★★★★ embroidery
You have in mind a companion for mylar and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mylar in all years. Click on graph for more.
mylar
white paint
vellum